1. Field of the Invention
The present invention relates to a forging apparatus, and more particularly to a forging apparatus for allowing a forged workpiece to be removed without the need for a draft angle.
2. Description of the Related Art
There has heretofore been known in the art a forging apparatus for forging a blank into a workpiece of given shape by placing the blank into a die cavity and pressing the blank with a press or the like (see, for example, Japanese Laid-Open Patent Publication No. 60-115342).
The forging apparatus has a die assembly comprising an upper die and a lower die which are vertically separable from each other and which, when put together, create a die cavity that is complementary in shape to the desired workpiece. In operation, the blank is placed on the upper surface of the lower die, and then the upper die is pressed against the blank by the press. When the blank is pressed between the upper and lower dies, the material of the blank flows and the blank is plastically deformed into a predetermined shape along the inner wall surfaces of the die assembly, thus producing the desired workpiece. Since the workpiece is forged under an intensive pressure by the press, strong pressing forces are applied from the contacting surfaces of the workpiece to the inner wall surfaces of the die assembly when the workpiece is forged.
When the forged workpiece is removed from the die assembly, it is difficult to separate the workpiece from the die assembly because of frictional forces generated between the inner wall surfaces of the die assembly and the forged workpiece. Generally, inner wall surfaces of the die assembly which extend substantially parallel to the direction in which the workpiece is pressed by the press have a draft angle, i.e., a taper, such that the inner wall surfaces are inclined at a certain angle so as to spread progressively in the direction in which the forged workpiece is to be pushed out or removed. Because of the draft angle of the die assembly, the forged workpiece also has a complementary draft angle which allows the workpiece to be easily removed out of the die assembly when the workpiece is pushed axially along the direction in which the inner wall surfaces spread progressively. The forged workpiece is formed to a size that is greater than the desired shape by the draft angle, and hence the blank from which the workpiece is forged is also of a greater size.
Since the draft angle on the workpiece is provided solely for the purpose of removing the workpiece from the die assembly, it is not a required intrinsic shape of the workpiece. Therefore, after the forged workpiece is removed from the die assembly, the workpiece is machined, e.g., ground, to flatten the draft angle into straight surfaces parallel to the axis of the workpiece, thereby achieving the intrinsic shape of the workpiece.
Inasmuch as the workpiece is subsequently machined, the workpiece needs to have a machining allowance for subsequent machining in the vicinity of the draft angle. Therefore, the workpiece is forged to a size including the machining allowance for subsequent machining.
The conventional forging apparatus has the draft angle for easy removal of the forged workpiece from the die assembly, and the forged workpiece is subsequently machined into the desired final shape separately from the forging process. Consequently, the process of manufacturing the workpiece comprises an increased number of steps, and hence is costly to perform.
In addition, as the workpiece has the draft angle and also needs to have the machining allowance in the vicinity of the draft angle, the amount of material of the blank to be forged into the workpiece is greater than required compared with the final intrinsic shape of the workpiece, resulting in an increase in the cost of the workpiece.
If the inner wall surfaces of the die assembly do not have a draft angle, but are straight surfaces, then the workpiece that is forged by the die assembly cannot easily be removed from the die assembly because of frictional forces generated between the inner wall surfaces of the die assembly and the forged workpiece.